Fog and skyboxes/skydomes etc. don't tend to work well together
On the other hand, since you've got a pretty fixed view you can make use of that.. You could add a black skyline to the background image slightly above where the buildings pop-up and put some black fog on the buildings. Or you could put some simple LOD in to reduce buildings to an untextured quad in the distance, and keep drawing them much further.
Theres a 'proper' way of doing good fog+sky, which is something like:
- draw all landscape into just the depth buffer to get the visibility info.
- Draw the background without any depth testing.
- draw all the landscape again, but with the fog setup to affect the transparency instead of a colour.
This way, you get the landscape fading into view from out of the sky
Look really good and probably worth it in this case..